FIG. 1 illustrates an example of a wireless infrastructure consisting of an access point (or a base station) and mobile stations communicating with each other via the access point. A power saving technique is applied to the mobile stations connected to the access point via wireless channels in order to reduce power consumption when they do not actually transmit and/or receive user data or traffic data. This type of wireless network structured by an access point (base station) and mobile stations connected to the access point via wireless channels is called a wireless infrastructure. In a wireless infrastructure, data transmission is performed directly between the base station and a mobile station, while data transmission between mobile stations 1 and 2 is performed via the access point.
In the wireless infrastructure (BSS) defined by IEEE 802.11, an access point or a base station transmits a common indication (beacon) at a prescribed frequency to mobile stations located under this access point. A beacon signal contains a variety of information items, including information about existence or non-existence of data packets for mobile stations operating in the power-saving mode in the transmission buffer of the access point, as well as identifications of the mobile stations located under the access point and the information about the base time. The information representing the existence or non-existence of the packets is called a traffic indication message (TIM). In the power-saving mode, a mobile station wakes up periodically according to the listen interval to receive the beacon signal. The state in which the mobile station is dormant and not receiving signals is called “Doze”. The state in which the mobile station is awake to receive the beacon signal is called “Awake”. The listen interval is a parameter for defining the beacon receiving frequency. The mobile station analyzes the TIM contained in the beacon to determine whether data for that mobile station are buffered in the base station. If there are data addressed to this mobile station in the access point, the mobile station transmits a control frame named a power saving poll (PS-poll) to the access point. In response to the PS-poll, the access point transmits data to the mobile station. Upon transmission of the PS-poll, the mobile station keeps “Awake” until the buffered data are received completely. If there are no data for this mobile station accumulated in the access point, the mobile station returns to the “Doze” state immediately. In this manner, the wireless mobile station in the power-wave mode repeats “Awake” and “Doze” at a prescribed cycle.
FIG. 2 illustrates a wireless communication system in which mobile stations relay data toward the destination without an access point (base station). This type of network is called an ad-hoc wireless network. The power-saving technique of intermittent reception may also be applied to the ad-hoc wireless network. In the ad-hoc wireless network, data transmission is performed directly between mobile stations (MS1-MS2, MS2-MS3, and MS3-MS4 in FIG. 2) located within the communication range, while indirect data transmission is performed between mobile stations that cannot directly reach each other (MS1-MS4, MS1-MS3, and MS2-MS4 in FIG. 2), via other mobile stations. The ad-hoc wireless network may be structured solely by wireless LAN equipment, such as notebook computers or cellular phones, as illustrated in FIG. 2, or it may be structured as an ad-hoc mesh network, as illustrated in FIG. 3, which network includes wireless LAN access points and wireless LAN equipment.
In an ad-hoc wireless network (IBSS) defined by IEEE 802.11, a signal named an announcement traffic indication message (ATIM) is transmitted to neighboring nodes operating in the power-saving mode to inform the nearby nodes that frames are to be transmitted in a while. The ATIM is transmitted in a specific time period starting from the beacon transmission time (which time period is called an ATIM window). The ATIM window only allows transmission of a beacon or an ATIM, and it requires the mobile stations to be in the “Awake” state. A node that receives an ATIM in the ATIM window maintains the “Awake” state until the frames are received, and a node that does not receive the ATIM returns to the “Doze” state. A node operating in the power-saving mode repeats the “Awake” state and the “Doze” state at a prescribed cycle.
Conventional wireless infrastructures and conventional ad-hoc wireless networks are described in publications listed below.                Patent-Related Publication 1: JP H5-183487A        Patent-Related Publication 2: JP 2005-57602A        Patent-Related Publication 3: JP 2004-234667A        Patent-Related Publication 4: JP 2004-336401A        Non-patent Publication 1: ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999Patent-related publication 1 discloses a technique for controlling the interval of intermittent reception in a wireless infrastructure. In this publication, a mobile station changes the listen interval or intermittent reception interval (i.e., the switching cycle between the awake state and the doze state) from the viewpoint of the frequency of sending and receiving messages, and reports the change of interval to the base station. In patent-related publication 2, a mobile station changes the intermittent reception interval by means of activated application software in a wireless infrastructure. Patent-related publication 3-discloses a method for controlling the power condition of the network interface module of a mobile station based on the information supplied from other modules. Patent-related publication 4 discloses a technique for saving power of an access point in a wireless infrastructure. In this publication, the access points shifts to the “Doze” state taking the traffic condition into account. Non-patent publication 1 describes that it is possible for a mobile station to change the interval of intermittent reception in a wireless infrastructure.        
With the conventional BSS, however, there is a problem in that the listen interval for intermittent reception is determined by a mobile station without considering the traffic conditions at the access point. If the listen interval set for intermittent reception at the mobile station is unnecessarily short, the mobile station wakes up too often even if no signal is to be transmitted from the access point. If the listen interval is set too long, packet transmission (reception) delay increases and the throughput may fall.
In the conventional IBSS, mobile stations (wireless nodes) in the network repeat “Awake” and “Doze” all at once at a prescribed cycle. Similar to the BSS structure, if mobile stations shift to the “Awake” state too frequently even if no signal transmission/reception is scheduled, power consumption increases unnecessarily. If the listen interval is too long, delay in packet transmission/reception increases and the throughput is reduced. These problems are likely to arise in both directions between mobile stations because there is no uplink/downlink distinction in IBSS, unlike BSS.
Although the above-listed patent-related publications and non-patent publication disclose techniques for changing the listen interval for intermittent reception, no publication proposes to set the listen interval for intermittent reception based upon actual traffic conditions. Therefore, the above-described problems are left without solutions.